One of several areas where thermoset resins are used is in molding compounds for composite systems. This includes sheet molding compounds (SMC) and bulking molding compounds (BMC). The composite systems such as fiber reinforced plastics are prepared on a compounding machine where fibers are incorporated into thickened paste. The paste typically comprises a resin, filler, initiator (for crosslinking), inhibitor, mold release agent, and thickening agent. Historically and currently, the primary thickening agents for sheet and bulk molding compounds have been magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2).
A key requirement for compounding and molding thermoset resins is controlling the viscosity throughout the production processes. The role of the thickening agent is to provide the required increase in viscosity of the resin paste mixture at a controlled rate during compounding, maturation (storage), and molding. For compounding, the viscosity should not exceed certain limits or the thickened resin paste mixture will not allow fiber wet-out. During maturation, the thickened fiber reinforced resin paste mixture will continue to increase in viscosity to the target molding viscosity. One challenge in using MgO and Mg(OH)2 thickeners is an upward drift in viscosity over time that will push the compound beyond the targeted molding viscosity, causing the material to be scrap. Conversely, if the compound is not thickened sufficiently the time required to reach molding viscosity could be excessive, e.g., over 72 hours, which affects production schedules detrimentally. Variations in raw materials (resins and filler) will also affect the thickening rate. Moisture is another challenging issue that affects the thickening reaction with MgO thickeners. This includes moisture in the raw materials and/or ambient moisture in the production facility.
In addition, for many polymeric articles of manufacture, performance characteristics including tensile properties are very important, these must also be balanced against processability requirements. Achieving a balance of excellent material and processing properties is a difficult endeavor and is complicated by the unpredictability of chemical interactions between various materials used in such compositions. It is particularly difficult to achieve an excellent balance of properties through compounding, maturation, and molding without adversely affecting the physical properties of the molded product.